The Clinical Mass Spectrometry Core collaborates with investigators from across the NIDDK Clinical Research Program, NIDDK basic research program and collaborating institutes to conduct stable isotope tracer studies (human and mammalian sources) and quantitative small molecule LC-MS and GC-MS. Clinical stable isotope studies include, but are not limited to, doubly labeled water, 6,6-D2-Glucose, D5-glycerol, 13C-palmitate. Doubly labeled water is used to measure total energy expenditure. Whereas the other labeled compounds are typically for metabolic studies. Metabolic studies with stable isotopes cover a vast range. Currently we measure labeled glucose, glycerol, and free fatty acids. The lab will advise, develop and validate novel research assays in collaboration with on-going clinical research projects and actively seeks new areas for collaboration with staff from across the NIH community. The most common projects nowadays involve mass spectrometry quantitation, by GC-MS but especially by LC-MS, of many drugs, metabolites, or small molecule compounds. Beside quantitation, we have identified unknown metabolite or even contaminants, which provided new avenues for investigation.